1. Field of the Invention
The present invention relates to a liquid jet head and a method of driving a liquid jet head, in which liquid is discharged by utilizing slip deformation of a piezoelectric body in a thickness direction to instantaneously change the volume of small spaces loaded with liquid.
2. Description of the Related Art
In recent years, an ink jet system liquid jet head has been used for creating characters and graphics by discharging ink droplets onto a recording sheet or the like, or forming a pattern of a functional thin film by discharging a liquid material onto a surface of an element substrate. In the ink jet system, ink or a liquid material is supplied from a liquid tank to the liquid jet head through a supply tube, and the ink is loaded into small spaces formed in the liquid jet head. In response to a drive signal, the volume of the small spaces is instantaneously reduced by utilizing an electrostrictive effect of the piezoelectric body to discharge liquid droplets from nozzles communicating to the small spaces.
FIG. 6A is a schematic cross-sectional view of a shear-mode liquid jet head 100. A plurality of grooves are formed in a surface of a piezoelectric substrate 101, and upper openings of the grooves are closed by a cover plate 106 to form a plurality of channels. The plurality of channels include discharge channels 102 for discharging liquid and dummy channels 103 having no liquid loaded thereto, which are arranged alternately with each other. The piezoelectric substrate 101 is subjected to polarization processing in a direction perpendicular to the surface thereof. Therefore, partition walls 107 are each polarized in the direction perpendicular to the substrate surface as indicated by the arrows of FIG. 6A. Common electrodes 104 are disposed on two side surfaces of the partition walls 107 on the discharge channel 102 side, which sandwich the corresponding discharge channel 102. Drive electrodes 105 are disposed on two side surfaces of the partition walls 107 on the dummy channel 103 side, which sandwich the corresponding discharge channel 102. The common electrodes 104 and the drive electrodes 105 are formed on the partition walls 107 in a portion above substantially half the height of the partition walls 107.
The common electrodes 104 formed on the two side surfaces of the corresponding discharge channel 102 are connected in common to a GND through a wiring electrode together with the common electrodes 104 of the other discharge channels 102. The two drive electrodes 105 disposed on the side surfaces of the dummy channels 103 on the discharge channel 102 side, which are adjacent to both sides of the corresponding discharge channel 102, are short-circuited through a wiring electrode, and connected to a terminal T for inputting a drive signal. When the drive signal is supplied to the terminal T, an electric field is applied in a direction orthogonal to the polarization direction of the upper half of the two partition walls 107, and hence the respective partition walls 107 slip to be deformed in a thickness direction to instantaneously change the internal volume of the discharge channel 102. In this manner, the liquid such as ink loaded into the discharge channel 102 is discharged from a nozzle 108.
However, when the liquid jet head is used over a long period of time, the drive signal of the same polarity is constantly applied in the direction orthogonal to the polarization direction, resulting in degradation of polarization P of the partition walls 107. In addition, the history of the applied drive signals differs among the discharge channels 102, and accordingly the degradation state of the polarization P also differs among the discharge channels 102. FIG. 6B schematically illustrates the polarization states of the respective partition walls 107 of the liquid jet head 100 after the long-term use, which are indicated by the arrows. The degradation state of the polarization also differs among the respective partition walls 107. Therefore, when the liquid jet head 100 is used with no measure taken, the liquid discharge condition becomes uneven, and consequently the recording quality decreases.
Japanese Patent Application Laid-open No. Hei 6-342946 describes the method of restoring the piezoelectric element made of a piezoelectric material to be used for an actuator or the like. In the description, a pellet piezoelectric element made of lead zirconate titanate (PZT) having a thickness of 0.5 mm is used, and after driving the piezoelectric element 107 times, an electric field is applied in a direction opposite to that of the drive electric field at a temperature of from 100° C. to 150° C., which is lower than the Curie temperature. Accordingly, the charged sites arranged by the application of the drive voltage are dispersed and broken, and an internal field is eliminated, with the result that the electromechanical coupling factor Kp and the mechanical quality factor Qm of the piezoelectric element are equalized to those of an unused product before the endurance test. Further, in the description, by subjecting the sample to polarization processing, the displacement amount and the polarization amount with respect to the applied voltage can be recovered substantially to the same state as that of the unused product.
Japanese Patent Application Laid-open No. 2002-355967 describes the drive apparatus capable of controlling the displacement unevenness of the piezoelectric element to be used for the bending-mode liquid discharge head. This liquid discharge head has a unit structure in which a pressure chamber loaded with liquid such as ink, an oscillation plate formed of an insulating film and a lower electrode, which is disposed on the pressure chamber, and a piezoelectric element formed of a piezoelectric thin-film layer and an upper electrode, which is disposed on the oscillation plate, are laminated one on another. The drive apparatus generates a drive waveform for driving the liquid discharge head having a large number of the above-mentioned pressure chambers arranged in parallel. Further, the drive apparatus generates a waveform for eliminating a remanent polarization of the piezoelectric thin-film layer. The remanent polarization changes with a lapse of time to cause unevenness between the elements. Therefore, the remanent polarization is eliminated by applying the waveform for eliminating the remanent polarization to the piezoelectric element. The waveform for eliminating the remanent polarization has a period of the same polarity as that of the drive waveform for driving the piezoelectric element, and an immediately succeeding period of an opposite polarity to that of the drive waveform. In the period of the same polarity, there is maintained a voltage level for applying an electric field intensity exceeding a coercive field of the piezoelectric thin-film layer, while in the period in which the polarity is reversed to the opposite polarity, there is maintained a voltage level for substantially applying the coercive field of the piezoelectric body. By applying the waveform to the upper electrode formed on the piezoelectric thin-film layer, the remanent polarization of the piezoelectric thin-film layer is set to 0. Accordingly, the change of the remanent polarization with a lapse of time can be prevented. The waveform for eliminating the remanent polarization is applied to the piezoelectric element at a timing immediately after powering on the printer, before or after cleaning the surface of the head, when replacing the ink cartridge, after delivering the paper, or other such timing than when discharging the ink.
Japanese Patent Application Laid-open No. 2006-68970 describes a method of restoring the piezoelectric element, which is a further improvement of the waveform described in Japanese Patent Application Laid-open No. 2002-355967. Specifically, between the period of the same polarity as that of the drive waveform, in which the voltage level for generating an electric field equal to or larger than the coercive field is maintained, and the period of the opposite polarity to that of the drive waveform after the above-mentioned period, in which the voltage level for generating the coercive field or an electric field equal to or larger than the coercive field is maintained, there is inserted a period of the opposite polarity to that of the drive waveform, in which a voltage having the absolute value smaller than the above-mentioned voltage of the opposite polarity is applied. Accordingly, the loads on the drive circuit and the piezoelectric element due to the steep change in potential are reduced.
In recent years, there has been increasing a demand for high-density arrangement of the discharge channels. In the case of the shear-mode liquid jet head, in order to achieve the high-density arrangement of the channels, it is necessary to reduce the thickness of the partition walls for partitioning the channels and the width of the channels in consideration of the structure of the liquid jet head. When the thickness of the partition walls and the width of the channels are reduced, the electric field intensity for driving the partition walls increases, and the polarization rotates due to the electric field applied in the direction orthogonal to the polarization direction, which raises the risk of degradation. Therefore, there is a demand for an effective measure to restore the degraded polarization of the piezoelectric partition walls.
In the method of restoring the piezoelectric element described in Japanese Patent Application Laid-open No. Hei 6-342946, the voltage is applied in the direction opposite to that of the drive voltage, and the charged sites dispersed and arranged by the application of the drive voltage are broken, to thereby eliminate the internal field due to the charged sites. In other words, because the charged sites need to be moved, the piezoelectric element is heated to the temperature of from 100° C. to 150° C. and the counter voltage is applied. When this restoration method is applied to the liquid jet head, there arises a need to separate and remove the piezoelectric element from the liquid jet head, or alternatively, there arises a need to heat the entire liquid jet head to 100° C. or higher, which complicates the restoration steps or disables the restoration steps from being carried out.
The piezoelectric element described in Japanese Patent Application Laid-open No. 2002-355967 or 2006-68970 is of the bending-mode type. Such a piezoelectric element has a structure different from that of the shear-mode type, and is driven by a different electric field. In Japanese Patent Application Laid-open No. 2002-355967 or 2006-68970, the piezoelectric thin-film layer has a thickness ranging from 1 μm to 3 μm, and the piezoelectric element is driven by an electric field sufficiently higher than the coercive field of the piezoelectric thin-film layer. In the shear-mode type, on the other hand, the piezoelectric body has a thickness at least one order of magnitude larger than that of the bending-mode type, and is subjected to the polarization processing. Such a piezoelectric element is driven by applying an electric field equal to or smaller than the coercive field. Accordingly, the degradation mode of the piezoelectric element is also different. In Japanese Patent Application Laid-open Nos. 2002-355967 and 2006-68970, the remanent polarization changes with a lapse of time, and the remanent polarization thus changed causes unevenness in the discharge condition. Therefore, the electric field of the same polarity as that of the drive waveform, which is at least twice as large as the coercive field, is first applied to the piezoelectric thin-film layer, and then the voltage of the opposite polarity, which is substantially equal to the coercive field, is applied, to thereby eliminate the remanent polarization that may cause the unevenness. In the shear-mode type, on the other hand, the piezoelectric body is polarized in advance, and slip deformation in the thickness direction is induced by applying the electric field in the direction orthogonal to the polarization direction. Therefore, when the remanent polarization is set to 0, the slip deformation in the thickness direction cannot be induced from the fact that the piezoelectric element is supposed to be driven by utilizing the remanent polarization. For this reason, the restoration method described in Japanese Patent Application Laid-open No. 2002-355967 or 2006-68970 cannot be applied to the shear-mode type.
Further, in Japanese Patent Application Laid-open Nos. 2002-355967 and 2006-68970, of the electrodes sandwiching the piezoelectric thin-film layer, the electrodes on one side are connected in common to be grounded, and the upper electrodes (individual electrodes) on the other side are individually connected to the drive circuit. To each of the upper electrodes, the drive voltage greatly exceeding the coercive field of the piezoelectric thin-film layer and the high reverse voltage having the polarity reverse to that of the drive voltage are applied. In other words, the drive circuit for driving the piezoelectric element needs to generate positive and negative high voltages, and hence the circuit structure is complicated, resulting in a large amount of load in constituting the liquid jet head.